Universal Serial Bus Motivation



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Signaling Environment


USB serial data is NRZI encoded prior to transmission

Non-Return to Zero Inverted


Signaling is differential

Helps to

Mitigate affects of noise on lines

Ensure data integrity



Configured as follows:




Detecting Device Attachment and Speed


Prior to transmission to client device

Host software must detect presence of device on bus

USB designed to automatically detect presence (absence) of device

When attached to (removed from) USB


Same mechanism used to determine speed of client device

Low speed or full speed



Detection


Detection accomplished by monitoring differential data lines

After cable power applied to port


When no device attached

Pull down resistors on D+ and D- ensure both data lines near ground

USB devices must provide pull-up resistor

On either D+ or D-

Based upon speed of device

D+ for full speed



D- for low speed
C
onnections appear as

When device attached

Current flows through voltage divider

Pull-down is 15 K 

Pull-up is 1.5 K

Signal on data line increases to about 90% of Vcc

When hub detects

One of data lines approaching Vcc

Other near ground

Knows device attached

By knowing which line is high

Can ascertain which kind of device has been attached

Thus when D+ or D- rises above 2.0 Vdc for greater than 2.5 sec

Device detected as attached


Removal detected when both lines

Fall below 0.8 Vdc for greater than 2.5 sec


When device attached or removed

Status bits set or reset to indicate state

Host software polls hub periodically to check for attachment or detachment

NRZI Encoding


Data transferred on USB

Encoded using NRZI encoding

Helps to ensure high data integrity without requiring clock

Such scheme has been used for many years


S
cheme shown as
Data

Transitions in data stream represent 0s

No transitions represent 1s

Encoder


NRZI encoder must maintain synchronization with incoming data stream

To correctly sample bits


Data stream must be sampled within data window

To detect whether transition has occurred since last bit time


Decoder


Decoder samples data stream during each bit time

To check for transitions


Transitions in data stream

Permit decoder to maintain sync with incoming data


Observe

Long series of consecutive 1s

Results in no transitions

Eventually receiver will loose sync


Solution

Use bit stuffing



Bit Stuffing


Forces transitions into NRZI data stream

When 6 consecutive 1s encountered

Ensures receiver detects transition

Every 7th bit time

Enables receiver to maintain sync
Transmitter is responsible for inserting 0 bit into data stream

Differential Pair Signaling


As noted using differential pair signaling

Noise reduction etc


As implemented in USB

Uses half duplex scheme

Device on either end can receive or transmit

Only one direction at a time

Half duplex scheme

Requires drivers be placed into high impedance state



When not transmitting data
Two single ended receivers also used to detect bus states
E
xample drivers look like the following


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